This proposal continues its focus on defining the mechanisms and receptors by which leukotriene E4 (LTE4), the most stable and abundant of the cysteinyl leukotrienes (cysLTs), promotes persistent airway inflammation and dysfunction. Using a combination of functional assays, biochemical approaches, newly created transgenic mouse strains, and two novel mouse models, we have begun to reveal the pathways by which LTE4 promotes airway eosinophilia and cytokine generation, and by which it induces bronchoconstriction. The central hypothesis of this application for renewal is that LTE4 elicits airway pathology by a hierarchical mechanism involving the activation of both mast cells (MCs) (directly through the novel LTE4 receptor GPR99 and platelets (indirectly through one or more MC products), the secondary generations of thromboxane A2 (TXA2), and prostaglandin D2 (PGD2), and stimulation of hematopoietic and non-hematopoietic T prostanoid (TP) receptors and chemoattractant homologous molecule expressed by Th2 cells (CRTH2) by these mediators, resulting in bronchoconstriction and amplification of inflammation. Aim 1 will identify the receptors that mediate pulmonary responses to LTE4 and identify the mechanisms responsible for their dysregulation in airway inflammation. Aim 2 will determine the respective contributions of MCs, platelets, and innate lymphoid type 2 cells (ILC2s) to the inflammatory and contractile effects of LTE4 in the airway. Aim 3 will determine the role of MC- and platelet- derived prostaglandins (PGs) in conveying the airway responses to LTE4, and identify their sites of action. These studies are expected to uncover potential therapeutic targets for asthma that are not addressed by currently available antagonists.